1. Field of the Invention
The present invention relates to a semiconductor sensor, such as an infrared-ray sensor, a gas sensor, an air fuel ratio sensor, a pressure sensor and an acceleration sensor, having a membrane structure.
2. Description of the Related Art
Conventionally, a semiconductor sensor having a so-called membrane structure is frequently used. A membrane (thin film) is formed, by etching, in the center of the undersurface of a wafer and a detecting element is formed on the membrane in order to increase the sensitivity of the sensor. FIG. 5A to FIG. 5C are illustrations showing a semiconductor sensor having a conventional membrane structure.
As shown in a perspective view in FIG. 5A, a semiconductor sensor 10 comprises an insulating material layer 20 and a substrate 30 made of silicon or the like for supporting the insulating material layer 20. As shown in a top view in FIG. 5B and in a sectional view along the A–A′ line in FIG. 5C, the undersurface of the substrate 30 is etched in the center of the sensor to form a thin film part 32 and then a membrane is formed by the thin film part 32 and the insulating material layer 20. After this, a detecting element (not shown) for sensing is formed on the top surface of the membrane so that it has a plurality of layers.
FIG. 6A to FIG. 6D are each a perspective view, a top view, a side view and a sectional view along the A–A′ line, respectively, showing a semiconductor sensor having a conventional membrane structure (membrane) in an assembled state. As shown schematically, the semiconductor sensor 10 is bonded to a mounting surface, such as a stem 50, with an adhesive 40. A sensor having such a membrane structure has been disclosed in, for example, Patent documents 1 to 3.
[Patent document 1]
Japanese Unexamined Patent Publication (Kokai) No. 7-58134 (paragraphs [0016] to [0020])
[Patent document 2]
Japanese Unexamined Patent Publication (Kokai) No. 6-129898 (paragraphs [0028] to [0038])
[Patent document 3]
Japanese Unexamined Patent Publication (Kokai) No. 7-120306 (paragraphs [0010] to [0024])
As shown in FIG. 6A to FIG. 6D, however, if the entire undersurface of the substrate 30 is bonded to the stem 50, a hollow part 34 under the membrane is sealed hermetically and a way of escape of a fluid, such as air, present inside the hollow part 34 is blocked. If the fluid expands or contracts within the hollow part 34 because of the change in temperature, a difference in pressure is produced between the inside and outside of the semiconductor sensor 10, an excessive stress is produced in the membrane, and deformation and distortion of the membrane occur.
In other words, as shown in FIG. 7A, when the fluid within the hollow part 34 expands, a gas pressure Pi inside the hollow part 34 becomes higher than a gas (atmospheric) pressure Po outside the hollow part 34, and the membrane made of the thin film part 32 and the insulating material layer 20 of the substrate 30 is deformed in such a way as to be pushed upward. In contrast to this, as the fluid within the hollow part 34 contracts, the gas pressure Pi inside the hollow part 34 becomes lower than the gas pressure Po outside the hollow part 34 and the membrane is deformed in such a way as to be pushed downward. If such deformation of the membrane is excessively large, there is a possibility of destruction of the membrane and if deformation of the membrane at a certain magnitude is repeated, there is a possibility of occurrence of fatigue failure of the membrane.
An example of a method for avoiding the above-mentioned problem is such one in which the bonding part of the bottom of the semiconductor sensor 10 to the stem is limited only to a part thereof (for example, refer to Patent document 1), as shown in FIG. 8A to FIG. 8D and FIG. 9A to FIG. 9E. FIG. 8A to FIG. 8D show how only opposite sides of the bottom of the semiconductor sensor 10 are bonded to the stem with adhesives 40a and 40b. FIG. 8A to FIG. 8D are each a perspective view, a top view, a side view in the X direction and a sectional view along the line A–A′, respectively. FIG. 9A to FIG. 9E show how only four corners of the bottom of the semiconductor sensor 10 are bonded thereto with the adhesives 40a, 40b, 40c and 40d. FIG. 9A to FIG. 9E are each a perspective view, a top view, a side view in the X direction, a sectional view along the line A–A′ and a sectional view along the line B–B′, respectively.
By limiting the bonding part only to a part of the bottom as described above, the hollow part 34 under the membrane and the outside of the semiconductor sensor 10 are communicated with each other through the gaps at the parts not bonded, as shown in FIG. 8C and FIG. 8D and in FIG. 9D and FIG. 9E and, therefore, a difference in gas pressure between the inside and the outside of the semiconductor sensor 10 can be eliminated.
However, the use of this method causes a problem: it becomes difficult to control the quantity of adhesive 40 to be applied when the semiconductor sensor 10 is mounted on the stem 50 and the mounting process becomes complicated. In other words, if the quantity of adhesive 40 to be applied is too small, the bonding strength is reduced and if, in contrast to this, the quantity is too large, the adhesive invades even unwanted parts and the hollow part 34 is sealed because the entire circumference of the bottom of the substrate 30 is bonded with the invaded adhesive. Therefore, it is necessary to precisely control the quantity of adhesive to be applied.
Another example method is such one as used in an infrared-ray sensor disclosed in Patent documents 2 and 3, in which the semiconductor sensor 10 is formed as a package and the internal space of the package and the hollow part 34 are put under a vacuum or in a pressure reduced state. FIG. 10 shows a conventional packaged infrared-ray sensor.
As shown in FIG. 10, the infrared-ray sensor 10 has an infrared-ray detecting part 80 formed on a membrane made of a thin film part 32 and an insulating material layer 20. The bottom of a substrate 30 is bonded to a stem 50 with adhesive 40 and a cap 70 having a window part provided with a filter 72 is fixed on the step 50 in a close contact (seal welded) state and thus the infrared-ray sensor 10 is formed as a package. Then, the inside of the cap 70 and the space within a hollow part 34 are put under a vacuum or in a pressure reduced state. By putting the hollow part of a semiconductor sensor 10 and the internal space of the cap 70 excluding the hollow part under a vacuum or in a pressure reduced state, as described above, the change in the difference in gas pressure between the inside and the outside of the semiconductor sensor 10 due to the change in gas temperature within the hollow part 34 can be eliminated or considerably reduced.
However, this method requires that packaging (forming as a package) be carried out in a hermetically sealed container equipped with an evacuation system which has a vacuum means and an exhaust means and, therefore, the process for assembling a sensor becomes complicated and the cost is raised.